Energy harvesting, wireless structural health monitoring system

ABSTRACT

A method of maintaining a structure includes providing a structure having a component subject to failure. A sensor, a memory and an energy harvesting device are mounted on the structure. The sensor is used and data derived from the sensor logged in the memory, wherein the memory is powered solely with energy derived from the energy harvesting device. The component is replaced if information in the memory shows that the component was subject to damaging usage.

RELATED APPLICATIONS AND PRIORITY

This application claims priority of Provisional Patent Application No.60/715,987, filed Sep. 9, 2005 and Provisional Patent Application No.60/798,570, filed May 8, 2006, both of which are incorporated herein byreference.

This application is related to the following commonly assigned patentapplications:

“Robotic system for powering and interrogating sensors,” U.S. patentapplication Ser. No. 10/379,224 to S. W. Arms et al, filed Mar. 5, 2003(“the '9224 application”), docket number 115-004.

“Wireless Vibrating Strain Gauge for Smart Civil Structures,” U.S.patent application Ser. No. 11/431,194 to M. Hamel, filed May 10, 2006(“the '194 application”), docket number 115-023.

“Sensor Powered Event Logger,” U.S. Provisional Patent Application No.60/753,481 to D. L. Churchill et al, filed Dec. 22, 2005, (“the '481application”) docket number 115-034.

“Slotted Bean Piezoelectric Composite,” U.S. Provisional PatentApplication No. 60/739,976 to D. L. Churchill, filed Nov. 23, 2005,(“the '976 application”) docket number 115-022.

“Method for Integrating an energy harvesting circuit into a PZ element'selectrodes,” U.S. Provisional Patent Application No. 60/753,679 to D. L.Churchill et al, filed Dec. 21, 2005, (“the '679 application”) docketnumber 115-035.

“Method for Integrating an energy harvesting circuit into a PZ element'selectrodes,” U.S. Provisional Patent Application No. 60/762,632 to D. L.Churchill et al, filed Jan. 26, 2006, (“the '632 application”) docketnumber 115-035a.

“Structural Damage Detection and Analysis System,” U.S. ProvisionalPatent Application No. 60/729,166 to M. Hamel, filed Oct. 21, 2005,(“the '166 application”) docket number 115-036.

“Energy Harvesting for Wireless Sensor Operation and Data Transmission,”U.S. Pat. No. 7,081,693 to M. Hamel et al., filed Mar. 5, 2003 (“the'693 patent”), docket number 115-008.

“Shaft Mounted Energy Harvesting for Wireless Sensor Operation and DataTransmission,” U.S. patent application Ser. No. 10/769,642 to S. W. Armset al., filed Jan. 31, 2004 (“the '642 application”), docket number115-014.

“Wireless Sensor System,” U.S. patent application Ser. No. 11/084,541 toC. P. Townsend et al., filed Mar. 18, 2005 (“the '541 application”),docket number 115-016.

“Strain Gauge with Moisture Barrier and Self-Testing Circuit,” U.S.patent application Ser. No. 11/091,224, to S. W. Arms et al., filed Mar.28, 2005 (“the '1224 application”), docket number 115-017.

“Miniature Stimulating and Sensing System,” U.S. patent application Ser.No. 11/368,731 to J. C. Robb et al., filed Mar. 6, 2006 (“the '731application”), docket number 115-028.

“Miniaturized Wireless Inertial Sensing System,” U.S. patent applicationSer. No. 11/446,637 to D. L. Churchill et al., filed Jun. 5, 2006 (“the'637 application”), docket number 115-029.

“Data Collection and Storage Device,” U.S. patent application Ser. No.09/731,066 to C. P. Townsend et al., filed Dec. 6, 2000 (“the '066application”), docket number 1024-034.

“Circuit for Compensation for Time Variation of Temperature in anInductive Sensor,” Reissue U.S. patent application Ser. No. 11/320,559to C. P. Townsend et al., filed Dec. 28, 2005 (“the '559 application”),docket number 1024-038.

“System for Remote Powering and Communication with a Network ofAddressable Multichannel Sensing Modules,” U.S. Pat. No. 6,529,127 C. P.Townsend et al., filed Jul. 11, 1998 (“the '127 patent”), docket number1024-041.

“Solid State Orientation Sensor with 360 Degree Measurement Capability,”U.S. patent application Ser. No. 10/447,384 to C. P. Townsend et al.,filed May 2003 (“the '384 application”), docket number 1024-045.

“Posture and Body Movement Measuring System,” U.S. Pat. No. 6,834,436 toC. P. Townsend et al., filed Feb. 23, 2002 (“the '436 patent”), docketnumber 115-002.

All of the above listed patents and patent applications are incorporatedherein by reference.

FIELD

This patent application generally relates to a system for structuralhealth monitoring and for health usage monitoring. It also relates tosensor devices and to networks of sensor devices with wirelesscommunication links. More particularly it relates to an energyharvesting system for providing power for monitoring structural healthand for transmitting data wirelessly.

BACKGROUND

Sensors, signal conditioners, processors, and digital wireless radiofrequency (RF) links continue to become smaller, consume less power, andinclude higher levels of integration. The combination of these elementscan provide sensing, acquisition, storage, and reporting functions invery small packages. Such sensing devices have been linked in wirelessnetworks as described in the '127, patent and in the '9224, '194, '481,'541, '731, '637, '066, and '436 applications.

Networks of intelligent sensors have been described in a paper,“Intelligent Sensor Nodes Enable a New Generation of MachineryDiagnostics and Prognostics, New Frontiers in Integrated Diagnostics andPrognostics,” by F. M. Discenzo, K. A. Loparo, D. Chung, A. Twarowsk,55th Meeting of the Society for Machinery Failure Prevention Technology,April 2001, Virginia Beach.

Wireless sensors have the advantage of eliminating wiring installationexpense and weight as well as connector reliability problems. However,wireless sensors still require power in order to operate. In some cases,sensors may be hardwired to a vehicle's power system. The wiringrequired for power defeats the advantages of wireless sensors and may beunacceptable for many applications. In addition, if a power outageoccurs, critical data may be lost, at least during the time of the poweroutage.

Most prior wireless structural monitoring systems have relied oncontinuous power supplied by batteries. For example, a paper “AnAdvanced Strain Level Counter for Monitoring Aircraft Fatigue”, byWeiss, Instrument Society of America, ASI 72212, 1972, pages 105-108,1972, described a battery powered inductive strain measurement system,which measured and counted strain levels for aircraft fatigue. Thedisadvantage of traditional batteries, however, is that they becomedepleted and must be periodically replaced or recharged. This additionalmaintenance task adds cost and limits use to accessible locations.

Given the limitations of battery power, there has been a need forsystems which can operate effectively using alternative power sources.Energy harvesting from vibrating machinery and rotating structures toprovide power for such sensing devices and for wireless networks ofsensors and/or actuators has been described in the commonly assigned'693 patent and in the '976, '679, '632, '642, and '731 applications.

A paper, “Energy Scavenging for wireless Sensor Networks with SpecialFocus on Vibrations,” by S. Roundy et al., Kluwer Academic Press, 2004,and a paper “Energy Scavenging for Mobile and Wireless Electronics,”Pervasive Computing, by J. A. Paradiso & T. Starner, IEEE CS and IEEEComSoc, Vol 1536-1268, pp 18-26, 2005, describe various strategies forharvesting or scavenging energy from the environment. These sensingsystems can operate truly autonomously because they do not requiretraditional battery maintenance.

However, these energy harvesting systems have not been optimized for useon structures, such as aircraft and for use in certain networks. Thus,an improved system for monitoring is needed that harvests sufficientenergy for operation and that can provide data over a network, and thissolution is provided by this patent application.

SUMMARY

One aspect of the present patent application is a method of maintaininga structure. The method includes providing a structure having acomponent subject to failure. A sensor, a memory and an energyharvesting device are mounted on the structure. The sensor is used anddata derived from the sensor logged in the memory, wherein the memory ispowered solely with energy derived from the energy harvesting device.The component is replaced if information in the memory shows that thecomponent was subject to damaging usage.

Another aspect of the present patent application is a method ofoperating a structure. The method includes providing a structure havinga component subject to component failure. The method also includesmounting a sensor module to the structure for measuring a parameterrelated to component failure, wherein the sensor module includes asensor, a wireless communication device and an energy harvesting device.Data is acquired with the sensor and information derived from the datais provided to the wireless communication device, the wirelesscommunications device is powered solely with energy derived from theenergy harvesting device and the wireless communication device transmitsthe information. The information is used to adjust operation of thestructure so as to avoid damaging usage.

Another aspect of the present patent application is a method ofoperating a structure. The method includes providing a structure havinga component subject to component failure; mounting a sensor, a memoryand an energy harvesting device on the structure; using the sensor andlogging data derived from the sensor in the memory, wherein the memoryis powered solely with energy derived from the energy harvesting device;and using information in the memory to adjust operation of the structureso as to avoid damaging usage.

Another aspect of the present patent application is a system comprising,a network of sensor nodes wherein each the sensor node includes asensor, a processor, a memory, a low power time keeper, a wirelesscommunication device and an energy harvesting device. The processor isconnected to receive data derived from the sensor. The memory isconnected for storing data derived from the sensor. The low power timekeeper connected to periodically provide a signal to wake the processorfrom a sleep mode. The wireless communication device is connected forcommunicating data derived from the sensor. The energy harvesting deviceconnected for harvesting energy to power the processor, the memory, andthe wireless communications device.

Another aspect of the present patent application is a structure,comprising a wireless instrumented structural component including afirst sensor, a second sensor, a processor, a transmitter, and an energyharvesting device. The first sensor is for measuring a first propertyrelated to structural load in the structural component. The secondsensor is for measuring a second property related to structural load inthe structural component. The first property differs from the secondproperty. The transmitter is connected to provide load data for thestructural component. All power for operating the transmitter is derivedfrom the energy harvesting device. The processor is connected to receivean output derived from the second sensor for verifying operation of thefirst sensor.

Another aspect of the present patent application is a method of using astructure, comprising providing a wireless instrumented structuralcomponent mounted to the structure, the wireless instrumented structuralcomponent including a first sensor, a second sensor, a transmitter, andan energy harvesting device. The first sensor is for measuring a firstproperty related to structural load in the structural component. Thesecond sensor is for measuring a second property related to structuralload in the structural component. The first property differs from thesecond property. Data from the first sensor is compared with data fromthe second sensor to determine that the first sensor is operatingproperly. Energy from the energy harvesting device is provided whereinall power for operating the transmitter is derived from the energyharvesting device. Data about structural load in the structuralcomponent is wirelessly transmitted.

Another aspect of the present patent application is a sensing device,comprising an inertial sensor and a GPS, the inertial sensor integratedwith the GPS, wherein all power for operation of said inertial sensor isprovided from energy harvesting.

Another aspect of the present patent application is a sensing systemcomprising a base station, a first plurality of sensors, and a secondplurality of sensors. The first plurality of sensors are connected tothe base station on a wired network. The second plurality of sensors areconnected to the base station on a wireless network.

Another aspect of the present patent application is a method ofestimating time before failure of a component of a structure. The methodincludes providing the structure having the component; instrumenting thestructure with a sensor and a memory, the sensor to measure a parameterrelated to the structure, the memory for logging data derived from thesensor; providing an energy harvesting device on the structure, theenergy harvesting device connected to provide all power for poweringlogging data; providing a model of the component subject to failure;entering information derived from the data into the model; and using themodel and the information to estimate a parameter related to time beforefailure of the component.

Another aspect of the present patent application is a method ofcollecting information about a structure, The method includes providingan instrumented component including a sensor, a memory, and an energyharvesting device, wherein the component is an integral part of astructure when installed in the structure, wherein the instrumentedcomponent includes packaging to protect the sensor, the memory, and theenergy harvesting device; installing the component in the structure; andusing energy derived from the energy harvesting device to provide powerfor logging data in the memory.

Another aspect of the present patent application is a method of ofmaintaining a structure. The method includes providing a structurehaving a component subject to component failure; mounting a sensor, amemory and an energy harvesting device on the structure; using thesensor and logging data derived from the sensor in the memory, whereinthe memory is powered solely with energy derived from the energyharvesting device; and logging data at a rate depending on amount ofenergy harvested by the energy harvesting device.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following detailed descriptionas illustrated in the accompanying drawings, in which:

FIG. 1 is a three dimensional view of a helicopter pitch linkinstrumented with energy harvesting, sensing, data storage and wirelesscommunications;

FIG. 2 is a three dimensional view of a triaxial inertial sensing suiteincluding sensors for measuring pitch, roll, and yaw;

FIG. 3 is a three dimensional view of a helicopter instrumented withenergy harvesting, data storage, wireless communications and sensors,including load and strain sensors on the pitch link, a shaft torque,RPM, and power sensing node mounted on the drive shaft of the rotor, aGPS that can be wired or wirelessly connected, wireless landing gearload sensor and a wireless airspeed sensor;

FIG. 4 is a block diagram of an energy harvesting, wireless structuralhealth monitoring system;

FIG. 5 is a block diagram of an energy harvesting sensing nodesubsystem;

FIG. 6 is another block diagram of an energy harvesting sensing nodesubsystem;

FIG. 7 is data collected in a simulated pitch link experiment;

FIG. 8 is a flow chart for fatigue and remaining lifetime calculations;

FIG. 9 is a three dimensional view of a field tunable vibration energyharvester for providing energy for a shipboard mounted wireless sensingnetwork;

FIG. 10 is a graph providing data showing the output power of a resonantflexure piezoelectric energy harvesting device in comparison withnon-resonant energy harvesting systems;

FIG. 11 is a block diagram of an inertial sensing system with triaxialaccelerometers, magnetometers, angular rate sensors and a GPS system;

FIG. 12 is a block diagram of a structural monitoring system gateway forintegrating data from a CAN bus and from an 802.15.4 wireless network;

FIG. 13 illustrates a star-mesh hybrid network topology;

FIG. 14 is a schematic diagram of one embodiment of the system of thepresent patent application;

FIG. 15 a is a cross sectional view of the mounting of the electronicsof the present patent application on a helicopter pitch link showingprotection from environment and mechanical stress; and

FIG. 15 b is a side view of the pitch link of FIG. 15 a.

DETAILED DESCRIPTION

Mathematical models, such as finite element models of a structure can beused to provide estimates of time before structural component fatigue.Such estimates are improved when their boundary conditions and inputloads are based on actual data obtained during operation of thestructure that may be collected by instrumented components mounted onthe structural component. Such smart components provide a significantbenefit in allowing engineers and owners of vehicles to obtain betterestimates of the remaining life of critical components.

Structural component fatigue information can be used for condition basedmaintenance (CBM). With CBM, parts are repaired, maintained, or replacedbased on their actual condition, rather than, for example, on the numberof total flight hours they may have logged or the number of miles theymay have been driven. CBM saves both time & money, since maintenancecrews only work on those parts that need maintenance, when they needmaintenance.

The data from smart components can also be used to reduce the cost ofspare parts inventories because spares are ordered based on their actualusage rates and the improved estimate of remaining life. Failures due tofatigue can also be avoided by timely replacing components that haveexperienced severe loads based on actual measurement of the live loadsand by continuous recording of the load histories and severity of usageover time.

In addition, warnings can be automatically issued to the operator of thevehicle when potentially damaging maneuvers are made, or when theoperator is subjecting the vehicle to severe usage. This warning can beused to train operators so that the wear and tear is reduced to theminimum required by the mission. This warning can also lead to saferoperation of the vehicle, and potentially to a reduced number offatalities and injuries to vehicle occupants.

The present applicants then provided various embodiments of a structuralmonitoring system capable of providing data including dynamic & staticvehicle strain, loading, temperature, location, and orientationinformation. The structural monitoring system they provide takes datafrom sensors, such as strain gauges, accelerometers, and thermocouplesrecorded along with data from inertial sensors, such as gyroscopes andaccelerometers. In addition data from the Global Positioning System(GPS) can be included. Both inertial sensors and GPS can provideaccurate pitch, roll, and yaw information about the vehicle. The variousembodiments include wireless communication, energy harvesting, andschemes for low power operation.

A way to obtain this structural monitoring information from a largenumber of sensors that can be deployed in arrays to provide informationover wide areas of a structure is provided. For example the sensors canbe placed at different positions on the vehicle and the sensors cancommunicate over a wired or over a wireless network. The network isscalable, meaning a large number of sensor nodes can be included in thenetwork.

A way to power each of the sensor nodes without requiring connection toa wall outlet and without having to periodically replace batteries isalso provided. Integrated in each of the sensor nodes can be a devicethat harvests energy from an available environmental source, such asvibration or strain energy. The integrated energy harvesting wirelesssensing nodes in the scaleable, wireless network provide substantialimprovement to previously available structural health monitoring systems(SMS).

Ways to substantially reduce power consumed by each of the sensor nodesso as to enable long battery life or so as to enable perpetual operationwith power available from energy harvesting are also provided.

The advances provided in the present patent application break downsignificant barriers to structural health monitoring. The scalablewireless network eliminates costly wire runs to strain gauges and othersensors and enable placing sensors on rotating parts and in other wiseinaccessible locations. Energy harvesting eliminates the need forbattery maintenance. Inertial sensors with GPS support providesimportant vehicle velocity, location, and orientation data, withenhanced accuracy compared to that of an inertial sensing system (ISS)alone. In addition, as further described herein below, the wirelessnetwork for external communications can be used in conjunction withwired communication to local sensors or other devices. In one embodimenta high speed networked wired CAN bus standard communication scheme isused to provide both power to inertial sensors and data acquisition fromthe inertial sensors using a multi-drop network.

The system architecture of the present patent application allows forflight tests to be performed with a range of wireless and wirednetworked sensing nodes. The wireless nodes may be deployed to monitorthe loads on the rotating components of helicopters, for example. Wiredand wireless nodes may also be used for fatigue monitoring onnon-rotating components, such a fixed wing aircraft. Other types ofaircraft, land vehicles, and water craft could also benefit from thecapability to autonomously track and assess structural damage “on thefly.”

Time stamped load data from key structural elements (collected by thewireless sensing nodes), combined with pitch, roll, and yaw information(collected by the ISS/GPS) is then gathered by an on-vehicle gateway.This allows time slices to be made through all the data, for example atpeaks in the inertial data when the stresses are greatest. This timesliced information can be used for flight regime recognition.

The present applicants found that by providing data logging and dataanalysis capability on-board, the vehicle becomes “self-aware” and canassess and record severity of its own usage and its usage history. Thisinformation can be used for condition based maintenance on every vehiclein a fleet, providing for example, information on the fatigue rates ofeach vehicle's structure and rotating components. If combined withcomponent tagging and tracking operators and maintenance and repairorganizations can use the SMS data obtained from actual severity ofusage and actual operating load measurements, as determined using thetechniques of the present patent application, to automatically updatethe status of the life-limited parts¹. The information can also be usedin health usage monitoring systems (HUMS). ¹ El-Bakry, M., ComponentTagging & Tracking—An Essential Enabling Technology for Effective ‘SafeLife’ Structural Monitoring, Proceedings of 5^(th) Intl. Workshop onStructural Health Monitoring, Stanford, Calif., Sep. 12-14, 2005

The present applicants have created working prototypes for a wirelessnetwork of sensor nodes, each of which harvests energy from availablestrain energy or vibration energy. The miniature, energy harvestingwireless nodes of the present patent application allow sensors to belocated in areas that are currently not instrumented, such as onrotating or moving components as well as in remote, inaccessible areas.The present applicants used both single crystal PZT and PZT fibers intheir energy harvesting prototypes. One system uses a tuned flexuralelement for vibration energy harvesting, while the other system harvestsstrain energy directly from a vibrating (cyclically straining) compositebeam². In both schemes, applicants demonstrated that sufficient energycould be harvested to power a wireless strain sensor transceiver³. Theyalso adapted their energy harvesting sensor systems for damage trackingon aboard helicopters and demonstrated that the operational strains inthe helicopter's control rod (or “pitch link”) generate enough power toallow continuous, wireless operational load monitoring of this criticalstructure, even during conditions of straight and level flight when theleast amount of strain energy is available for harvesting.⁴ ² Churchill,D. L., Hamel, M. J., Townsend, C. P., Arms, S. W., “Strain EnergyHarvesting for Wireless Sensor Networks”, proc. SPIE's 10th Int'lSymposium on Smart Structures & Materials, San Diego, Calif., paperpresented March 2003³ Arms, S. W., Churchill, D. L., Townsend, C. P.,Galbreath, J. H.: “Power Management for Energy Harvesting WirelessSensors”, proc. SPIE's Symposium on Smart Structures & Materials SanDiego, Calif. March 2005⁴ Arms, S. W., Townsend, C. P., Churchill, D.L., Moon, S. M., Phan, N., “Energy Harvesting Wireless Sensors forHelicopter Damage Tracking”, accepted for presentation at AHS 2006,Health & Usage Monitoring Systems (HUMS), Phoenix, Ariz., May 9-11, 2006

Pitch links are critical rotating elements on helicopters, and are verydifficult to monitor with existing technologies, such as slip rings.Pitch link loads in the Sikorsky H-60 have been found to vary stronglywith flight regimes: during pull-ups & gunnery turns, the loads weremeasured at approximately eight times that of straight & level flight⁵.⁵ S. Moon, D. Menon, G. Barndt, Fatigue Life Reliability Based onMeasured Usage, Flight Loads, & Fatigue Strength Variations, Am.Helicopter Society 52nd Annual Forum, Washington, D.C., Jun. 4-6, 1996

Therefore, pitch link loads are a good indicator of helicopter usageseverity. Pitch link loads are high, typically in the range of 1600 lbsin static compression accompanied by +/−7800 lbs of cyclic (dynamic)loading. The present applicants found that strain gauges placedstrategically on the pitch link can directly measure both static anddynamic loads, while canceling out thermal errors. They also found thatthey could use energy harvesting methodologies to convert the pitchlink's dynamic strains into power to completely eliminate the need forbattery maintenance.

A representational image of the energy harvesting pitch link 30 withsurface mounted strain/load sensors 32 and support instrumentation 34 isprovided in FIG. 1. Environmental protection for sensors 32 andinstrumentation 34 mounted on pitch link 30 is shown as transparent inthis illustration. Sensors 32 includes piezoresistive strain gauge 36.Instrumentation 34 includes circuit board 38 that includesmicroprocessor 40, rechargeable electrochemical battery 42, and RFtransceiver 44. Instrumentation 34 also includes piezoelectric energyharvesting elements 50, and RF antenna 52. Instrumentation 34 can beprotected with electrical insulation, EMI shielding and a protectivecover, not shown.

In order to perfect a sensing solution which exploits energy harvesting,the power consumed by all of the system's components (sensor,conditioner, processor, data storage, and data transmission) must becompatible with the energy harvesting strategy and the available powerlevels it can provide. Obviously, minimizing the power required tocollect and transmit data correspondingly reduces the demand on thepower source. Therefore, minimizing power consumption is as important agoal as maximizing power generation. The present applicants havesuccessfully minimized power consumption by 1. placing the sensing nodesin micropower sleep mode as much as possible, 2. reducing the durationand frequency of radio frequency (RF) communications, and 3. reducingthe sensor's sampling rates to the minimum required by the application.

Integrated inertial & magnetic sensor triaxial suite 60, called3DM-GX1®, available from Microstrain, Inc., Williston, Vt.http://microstrain.com/3dm-Ex1.aspx is illustrated in FIG. 2. This3DM-GX1® device combines three angular rate gyros with three orthogonalDC accelerometers, three orthogonal magnetometers, multiplexer, 16 bitA/D converter, and embedded microcontroller, to output its orientationin dynamic and static environments.

Operating over the full 360 degrees of angular motion on all three axes,3DM-GX1® provides orientation in matrix, quaternion and Euler formats.The digital serial output can also provide temperature compensated,calibrated data from all nine orthogonal sensors at update rates of 350Hz.

Networks of 3DM-GX1® nodes can be deployed by using the built-in RS-485network protocol. Embedded microcontrollers relieve the host system fromthe burden of orientation calculations, allowing deployment of dozens of3DM-GX1® nodes with no significant decrease in system throughput.

Output modes and software filter parameters are user programmable.Programmed parameters and calibration data are stored in nonvolatilememory.

Using this device, data concerning structural performance can becollected and transmitted without human intervention and without needfor connectivity to the vehicle's flight computers.

An embodiment of a wireless sensing system combined with inertialsensing and GPS to provide a flexible monitoring system for helicopter70 is illustrated in FIG. 3. The system includes wireless energyharvesting load and strain sensors 72 mounted on the pitch link 74 ofhelicopter 70 along with drive shaft torque, RPM, and power sensors 76.Other sensors, such as an inertial sensing suite of triaxialaccelerometers, rate gyros, and magnetometers, with GPS input 78 and GPSantenna 80 can be deployed. Wireless landing gear load sensing 82 andwireless airspeed sensing 84 can be deployed as well.

The sensors can have wireless network connection or vehicle bus networkconnection.

The miniature electronics modules are designed to support sensing and RFcommunications at microwatt energy levels. This enables their use withstrain and vibration energy harvesting systems the present applicantshave demonstrated. The present applicants have also demonstratedwireless sensing nodes that support other sensors, such as conventionalstrain gauges and thermocouples⁶. ⁶ Arms, S. W., Townsend, C. P.,Churchill, D. L., Moon, S. M., Phan, N., “Energy Harvesting WirelessSensors for Helicopter Damage Tracking”, accepted for presentation atAHS 2006, Health & Usage Monitoring Systems (HUMS), Phoenix, Ariz., May9-11, 2006

The strain gauge nodes are capable of peak valley compression andfatigue calculation using embedded rainflow algorithms⁷. Thisversatility allows the wireless sensing network to be tailored to bestmeet an aircraft's specific monitoring requirements, and facilitatestheir use on aging aircraft, where it is best not to disturb theexisting wiring. Furthermore, the nodes' embedded software may bewirelessly upgraded to allow enhancements to the damage detectionalgorithms and structural interrogation protocols in the future. ⁷ Arms,S. W., “Scaleable, Wireless Structural Testing System”, AerospaceTesting Expo 2005 North America, Open Technology Forum, Long Beach,Calif., Nov. 7-11, 2005

Standard open architecture communications for an embodiment of astructural health monitoring system is shown in the block diagram inFIG. 4. This system provides support for hard-wired digital CAN bus 90for networked sensors and actuators 92, 94 and GPS receiver 96. It alsoprovides support for digital wireless communications for other sensors,such as wireless strain, vibration, torque and load sensors 98, whichobtain their power for acquiring, processing, and transmitting data fromenergy harvesting. Wirelessly communicating airspeed sensor 100, canalso submit its data. Both the wired and the wireless sensors andactuators are connected to base station 102 which can include aprocessor and/or an on-board digital signal processor 104 which runsstructural usage algorithms. Also connected to base station 102 areflash EEPROM 106 for logging data, IEEE 802.15.4 transceiver 108 anddisplay 110 which can be located, for example, in the cockpit of theaircraft. To get a measure of what the flight controls of an aircraftare attempting to make the aircraft's control surfaces do informationfrom servo actuators 112 is provided to base station 102. From vehiclebus 114 a large amount of data can be provided to base station 102,including engine temperatures and pressures, and remaining fuel.

Operating requirements for the wireless sensing node's are listed inTable I.

TABLE I Wireless sensing node Specifications Parameter Typical UnitsOperating temperature range −55 to +85 Degrees C. On board temperature−55 to 85   Degrees C. measurement range Humidity range 0 . . . 100 % RHDifferential sensor inputs 3 bridge sensors inputs n/a (supports sensortypes including strain gages, accelerometers, pressure sensors, & loadcells), 1 temperature sensor Differential input gains Softwareprogrammable V/V    10-10,000 Differential input offset adjust Softwareprogrammable mV referred +/−100 to input Sensor Excitation DC 3.0 V/50mA n/a maximum RF Transmission Frequency 2.450-2.490 GHz RF channels 16n/a RF Transmission range 70 Meters (line of sight) RF Output power 0dBm (1 mW) n/a RF Modulation Type Direct Sequence Spread n/a SpectrumWireless Data standard IEEE802.15.4 n/a Data security encryption AES-128n/a Data acquisition resolution 16 bits Max data acquisition rate 4000samples/second Data Storage on standard 8 Megabytes board Flash Powerrequired to maintain 2.4 microwatts precision time stamping capabilityPower required to maintain 3 microwatts microprocessor sleep timer PowerSupply min/max 3.1/40 Volts DC Mechanical dimensions of <1.0 × 1.0inches WSN microelectronics

The wireless sensing node system is a network of wireless energyharvesting miniature sensing elements that allow for acquisition &storage of Wheatstone bridge type sensor data in a package size lessthen 0.5 cubic inch. For example, vibrating energy harvesting wirelessstrain gauge electronics integrate the following electronic functionalblocks into a microminiature package smaller than 1.0 cubic inch involume, including

a) energy harvesting power conversion & storage electronics

b) programmable precision triaxial strain gauge signal conditionerw/integral self calibration

c) low power system microprocessor

d) IEEE802.15.4 direct sequence spread spectrum radio transceiver

e) Flash memory for local data logging

f) Nanopower time keeper to activate scheduled datalogging modes

Each wireless sensing node 120 includes or interfaces directly toWheatstone bridge type sensors 122 through programmable sensor signalconditioning 124, multiplexer, instrumentation amplifier, andanti-aliasing filter 126, and A/D converter 128, as shown in the blockdiagrams in FIGS. 5, 6. Microprocessor 130 manages the data acquisitionand storage of data into nonvolatile flash memory 132. WirelessIEEE802.15.4 transceiver 134 is provided for real time data transmissionor data download after completion of the test.

Embedded microprocessor 130 provides the programmable intelligence tomanage data acquisition from embedded sensors 122. Embeddedmicroprocessor 130 will also store configuration and calibration datafor the individual sensors and data acquisition channels. As shown inFIG. 6 multiple data acquisition channels can be provided withmultiplexing.

The system supports both high level voltage inputs (0-3 Volt inputs) andlow level inputs (<100 mV, typical) from sensors, such as bonded foilstrain gauges. To support low level inputs a programmable sensorinterface (PSI) front end is provided. This front end uses high commonmode rejection ratio (CMRR) instrumentation amplifiers to convert thelow level voltages produced by most sensors to a high level voltagesuitable for the system analog to digital converters (ADC). Theseinstrumentation amplifiers have a digitally programmable gain (DPG) anddigitally programmable offset (DGO), allowing any type of sensor thatproduces an analog voltage output to be used, such as strain gauges,thermocouples, pressure sensors, and accelerometers. Following theinstrumentation amplifiers are digitally programmable low pass filters.These filters are used for noise reduction and will also serve as antialiasing filters for the analog to digital converters. The user is ableto adjust all PSI parameters using a software graphical user interface(GUI). Once the appropriate settings have been selected they will bestored in non-volatile memory on the microprocessor. The systemmicroprocessor manages the initialization of all the digitallyprogrammable parameters on startup.

In addition to programmability, the system also has the capability toperform embedded built in test and calibration of the electronics. Tofacilitate built in test and calibration analog switch 140 is used toswitch in known test signals to the device inputs under microprocessorcontrol, allowing the instrument to calibrate both its offset and itsgain without user intervention. For the instrumentation amplifiers, thereference calibration signal is accomplished by switching in a knownresistance across one leg of the bridge circuit of sensor 122. Switchingin a known resistance produces a known voltage shift at the input of theinstrumentation amplifier, allowing the system to determine the scalefactor of PSI circuit. For the high level 0-3V channels a knownprecision reference voltage is switched into the input. System offsetwill be obtained and calibrated by using analog switches to temporarilyshort the inputs together and measuring the output offset voltage. Oncethis system gain and offset data is obtained, it will be saved into nonvolatile memory on the system microprocessor for use when sensor data iscollected.

The system uses digital to analog converter (DAC) 142 to provideexcitation to the sensors for the calibration. The voltage levelsprovided by the DACs will be programmable between 0 and 3 volts, andwill be able to source up to 50 mA of source current to the sensor load.

Signal Wireless Data Acquisition & Logging of Sensor Data

After the signal from sensor 122 has been amplified and filtered, thesignal will be acquired using high speed analog to digital converter(ADC) 128 with programmable conversion rates as high as onemegasample/second. The output of this converter is stored in memoryusing direct memory access (DMA), allowing the high speed capability ofthe ADC to be preserved. For the highest speed acquisition, the data isfed directly into static random access memory (SRAM) for the duration ofthe test. The use of SRAM is desirable as it supports high speedacquisition at relatively low power. After the test is completed, thedata can be transferred to non-volatile flash memory 132 and can bedownloaded via a wired or wireless interface at a later time.Alternatively, that data can be transmitted in real time over thewireless IEEE802.15.4 interface.

One embodiment uses a memory chip that integrates one megabyte of SRAMand two megabytes of flash memory into a package that measuresapproximately 6 mm×8 mm. The 16 bit ADC with DMA is integral to thesystem microcontroller (C8051F061, Silicon Labs, Austin, Tex.). Thesample rate can be programmable by the user from 100 Hz to 1 MHz. Theamount of time that the sensor data is acquired by the burst samplingmode can also be programmable by the user. For a triaxial rosette straingauge and a flash size of 2 Megabyte, the present applicants found thatdata can be acquired and stored with burst mode sample rates of 100 kHzfor up to 3.4 seconds. For a 100 Hz acquisition rate, burst mode can becontinued for 3400 seconds (˜56 minutes) before memory is filled anddata is download.

Triggering and Time Synchronization

Triggering data acquisition from the sensors can be through a commandover wireless IEEE802.15.4 network 134. For applications that requiretime synchronization, the trigger packet can include network timesynchronization data. For many embedded applications the sensed dataacquired by individual structural health monitoring modules can besynchronized in time. Each wireless sensing node module has precisiontime clock 146 that can be periodically resynchronized over wirelessnetwork 134. The wireless synchronization method will support timesynchronization between remote wireless sensing node modules 120 to aresolution of better than 1 millisecond.

Base station 102 and the inertial sensing suite can also have a realtime precision time clock. Further timing information can be derivedfrom the GPS unit which is connected to the inertial sensing suite. Thistiming information can then be broadcast to the network.

Testing synchronization can be accomplished by providing the same inputto each node 120 of multiple wireless sensing node breadboard nodesusing a function generator producing a 10 Hz sine wave. Data acquisitionis triggered using the trigger data packet, and data collected andstored locally along with a time stamp for each data point. The timestamp is initialized to zero on detection of the synchronization packet.The data is downloaded, and phase lag and synchronization betweenchannels can be documented. The time stamp can include the calendar dateand time. The present applicants found that they could provide thisinformation while maintaining extremely low average quiescent currents.

Micropower Timekeeper & Timed Burst Mode Sampling Capabilities

Micropower time keeper 146 is included in the wireless sensing nodesystems to provide scheduled sampling of sensor data (or scheduled wakeup) during conditions of low vibration (or machine downtime). Micropowertime keeper 146 (Maxim DS1390, Sunnyvale, Calif.) draws less than 800na. A small button cell battery may be included to power micropower timekeeper 146 to ensure its operation after extended periods of lowvibration where energy storage elements may be completely discharged.Thus, timing can be preserved even under such conditions. Alternatively,time can be broadcast to each wireless sensing node. A serial interfacefor this component for connection to a processor is well known.

In lowest power mode, power to all circuitry on the wireless sensingnode printed circuit board (PCB) (except micro power time keeper 146) iscompletely turned off. DS1390 timekeeper 146 may be programmed to createa single pulse interrupt at a time interval that has been preprogrammed.This interrupt line is routed up to one of the microprocessor'sinterrupt pins. Upon interrupt, processor 130 wakes up, and depending onthe preprogrammed instruction set, it executes one or both of thefollowing:

(1) The wake up may initiate a “sniff” for the presence of the 802.15.4carrier. If the carrier is present the wireless sensing node programwill cause the system to enter into communication mode. The system canthen, depending on the communicated commands, cause one or more of thefollowing to happen; the wireless sensing node system will sendpreviously stored data via the 802.15.4 radio link to the requestingbase station, and/or the system will accept and self program newparametric data from the requesting base station which can includeparameters for the number of data points to be acquired during eachsampling, the sampling rate, how many channels to sample, the intervalbetween sampling sessions or the times of day to sample (scheduledsampling), and new gain or offset parameters for the programmable signalconditioners.

(2) The wake up may initiate a data acquire and store sequence. Thissequence includes powering up the wireless sensing node acquisitionPCB's and starting data acquisition according to a preprogrammedsequence that ends in the data acquired being stored in electricallyerasable programmable memory (EEPROM).

After any of the above sequences are completed, wireless sensing nodemodule 120 will power off the acquisition circuitry and then enter sleepmode itself until the next scheduled interrupt occurs. During sleep modethe energy storage elements shall be background recharged by energyharvesting system 148.

Sampling Modes: To maximize flexibility, vibrating energy harvestingwireless sensor node 120 may support a number of sampling and storagemodes, including:

a) real time streaming of wireless strain readings at programmable datarates (from 0.1 Hz-1000 Hz)

b) “burst” mode sampling and storage at very high sample rates (up to 1MHz) at scheduled time intervals, to be downloaded at a later time.

In the real time streaming mode, sensor 122 is sampled at a fixed rateand the data is transmitted over RF link 134. At low update rates, thepower consumption required to do this is extremely low. For example, thepresent applicants have demonstrated that to sample and transmit datafrom a 1000 Ohm strain gauge requires just 275 microamps of averagesupply current (<825 microwatts power) at 3 volts DC when sampling thestrain gauge at 40 times per second and transmitting the block of dataonce per second. As the sample rate increases, the required power alsoincreases.

Burst mode sampling allows for the system to support very highdatalogging rates for short periods of time. If the burst mode samplingevents are executed at a low duty cycle, then the average power is quitelow. Depending on the amount of energy available the system can beprogrammed to adjust its sampling rate appropriately. For example duringconditions when little energy is available to harvest sampling rates canbe reduced or sampling can be stopped and available energy can be usedto background recharge until such time as enough energy has been storedto take a sample. Alternatively, if the power produced by the energyharvester increases then sampling rate can be automatically increased.

The present applicants found several ways to reduce power consumption.Increasing the bandwidth or frequency response of the sensor signalconditioning chain allowed for faster settling of the sensor signalconditioner, so it can be turned on and off quickly, which minimizes thetime required for the sensor to be powered, and hence lowers the overallpower requirement of the sensor. Thus, using higher power components canresult in energy savings.

Adding a burst mode sampling feature which buffers data for one secondprior to sending the data also reduced power consumption. The presentapplicants found that significant power is used in sending a RF datapacket using the standard IEEE802.15.4 packet protocol and advantagefrom sending one large data packet compared to sending many smallerpackets. By buffering and reducing the number of data packets sent persecond, they found that power consumption could be greatly reduced. Inone sampling mode implemented, the sensor data was digitized at a 40 Hzrate and buffered for one second before data was transmitted to thereceiver.

Optimizing the power up sequencing of the amplifiers in the analogsignal chain also reduced power consumption, the present applicantsfound that the analog electronics signal chain would settle faster whenthey kept the amplifiers out of saturation during their warm up period.They optimized firmware to ensure that the power up sequence minimizedthe time that the amplifiers were in saturation. For example, if theamplifiers are powered after the sensors the amplifiers are kept out ofsaturation or the time minimized.

These changes resulted in reducing the power consumption from 21,000microwatts in our previous low power sampling mode to 975 microwatts, agreater then twenty-fold reduction in power consumption.

The wireless strain node was then tested with PZT elements as its solepower source and with an electrodynamic actuator generating simulatedpitch link cyclic strains. A conventional 1000 ohm foil type straingauge (Visay Micro-Measurements), was bonded to the pitch link testspecimen. The device was found to be able to sustain sampling andtransmitting of strain data at the desired 40 Hz rate for pitch linkcyclic strain levels above +/−100 microstrain. Representative data inFIG. 7 shows that sufficient energy is generated to operate theelectronics indefinitely without batteries from normal operation of apitch link on a helicopter.

Algorithms were also developed for peak valley compression and rain flowfatigue calculations, as shown in the flow chart in FIG. 8. Embeddedwithin the memory of each energy harvesting wireless strain sensingnode, the algorithms further reduce power consumption and enable thewireless nodes to track accumulated damage. Power consumption is reducedsince performing calculations on board reduces the amount of wirelessdata communications which is a large power consumer. The measurement ofaccumulated damage can be used to optimize machine maintenancescheduling and to predict and prevent failures.

Strain data is acquired from sensors, as shown in box 150. A peak-valleyfilter is applied to this data to simplify the data into peaks andvalleys, as shown in box 151. This reduces the amount of data that needbe stored. Then a small cycle filter is applied, as shown in box 152,that removes the peaks and valleys that are below a programmable smallexcursion threshold. Then a component or structural model is applied tothe filtered strain to adjust to accommodate strain concentrations orother geometrical considerations that might amplify or attenuate strainin another location on the structure or the component, as shown in box153. Next the amplitude of each peak-valley strain cycles is determinedusing a rainflow algorithm, as shown in box 154. Minor's rule isapplied, as shown in box 155 along with the embedded S/N curve in box160 and the strain amplitude from box 154. Minor's rule provides thatthe current life used is equal to the previous life used +1/N wherein Nis the estimated number of cycles that would produce a failure at aspecified strain level. The number of cycles remaining before failurecomes from the embedded S/N curve and the amplitude measured in rainflowalgorithm box 154. The output of box 155 is the life used. Remaininglife is then computed in box 156 from the hours the vehicle has beenused divided by the life used as calculated in box 155 minus the hoursused, where the hours used are derived from a real time clock and sensedparameter, as shown in box 157. The remaining life estimate from box 156can then be provided on demand or wirelessly transmitted to maintenancepersonnel as shown in box 158 and a database updated for that componentand for associated components, as shown in box 159.

For an application such as the pitch link, strain readings can easily beconverted to loads through a prior calibration step applying staticloads to the the pitch link and measuring its strain response. Amathematical relationship between strains and loads can also be used.

Monitoring live service loads of pitch links is important for healthusage structural monitoring because the pitch link is a key structuralmember connecting a helicopter's rotor blade pitch horn with therotating swash plate. The present patent application providesinstrumented pitch links that allow for improved characterization. Asshown in FIG. 1 and FIGS. 5 and 6 the instrumented wireless energyharvesting pitch link of the present patent application integratessensing, data acquisition, energy harvesting, energy storage, datalogging, and wireless communications elements while eliminating batterymaintenance and wiring for power or communications, allowing it to belocated on such places as rotating components.

Energy Harvesting and Background Recharging for Wireless Sensing Modules

Vibration or strain energy may also be harvested using low costpiezoelectric (PZT) materials. A custom, tapered mechanical structuredescribed in commonly assigned copending U.S. Provisional PatentApplication No. 60/739,976, “Slotted Beam Piezoelectric Composite,” toDavid Churchill, incorporated herein by reference, efficiently convertslow level vibrations to high strains, and serves as the carrier for thePZT material that converts the strains into electricity.

The harvester's resonant frequency can be tuned in the field usingexternal adjustment 150 by moving location of the proof mass or byadjusting a magnetic field in proximity to a ferrous material or apermanent magnet located on the mass, as shown in FIG. 9 and asdescribed in the '976 application. The PZT material was epoxy bonded tothe tapered flexure element, which provided a uniform strain field tothe PZT material as described and illustrated in the '976 application.During tests of this element, a strain gauge was bonded to the PZTmaterial to facilitate documentation of the strain levels that werepresent within the PZT. Adjustment of the (240 gram) proof mass locationrelative to the fixed end of the flexure allowed us to mechanically“tune” the harvester's resonant frequency. Resonance could be readilyadjusted from about 38 Hz to about 55 Hz.

The weight of the vibrating energy harvesting wireless sensor issignificantly lower than the weight of a conventional hardwiredinstallation of similar capability. It is also low enough so as not tosignificantly influence the resonant frequency of the underlyingstructure to which it is mounted. While the allowable mass depends onthe specific structure to which it is attached the present applicantsfound that components weighing less than several ounces meet thesecriteria. Since the proof mass is the single largest contributor tosystem weight, they found that providing a proof mass of 2 ounces orless allows for a target weight for a complete vibrating energyharvesting wireless sensor of 3-4 ounces.

Harvester Design: Analytical modeling performed by the presentapplicants has shown that, for a given input vibration frequency, w, anddisplacement amplitude of vibration, A, the electrical power, U, thatcan be generated by a vibrational energy harvester is estimated by

$U = \frac{{m \cdot \zeta}\; {e \cdot A^{2} \cdot \left( \frac{\omega \; n}{\omega} \right) \cdot \omega^{3}}}{\left\lbrack {\left( \frac{\omega \; n}{\omega} \right)^{2} - 1} \right\rbrack^{2} + \left\lbrack {2 \cdot \left( {{\zeta \; m} + {\zeta \; e}} \right) \cdot \frac{\omega \; n}{\omega}} \right\rbrack^{2}}$

where m is the proof mass, ζe is the electrical damping, ζm is themechanical damping, and ωn is the natural frequency of the harvester'sresonant structure. Thus, the power output is proportional to themagnitude of the proof mass and to the square of the vibrationamplitude.

For helicopter applications, small energy harvesters can be used sincethe vibration levels tend to be high aboard helicopters. A vibrationharvester can be used for providing power for applications such asmonitoring the vibration conditions of gearboxes and recording thermalhistories of engine compartments and electronic bays.

In the case of the pitch link of a helicopter, the present applicantshave demonstrated that background battery recharging can be accomplishedusing the direct strain energy harvested from the straining pitch linkitself, eliminating the need for a resonant beam element. Thus, wirelessenergy harvesting load sensing of helicopter pitch links can rely onpitch link strain energy harvesting. The wireless sensing subsystem usedon the pitch link is scalable to allow support of a wide range ofsensing and energy harvesting applications.

Charge generated by the strained piezoelectric material was stored on aninput capacitor. Once sufficient charge had accumulated on the inputcapacitor, and the voltage on this capacitor reached a prescribed level,then the output of this capacitor was fed to the input of a highefficiency step down converter. The converter steps the high voltage onthe input capacitor to a lower voltage on the output capacitor (orbattery) to provide power to drive the wireless sensing node, asdescribed in the '693 patent and in the '642 application. In order toprovide for long term energy storage, a stack (of five), thin filmelectrochemical batteries (Infinite Power Solutions, Golden, Colo.) wereused in place of the output capacitor. These batteries are advantageousbecause they exhibit extremely low leakage, they are very thin, and theydo not suffer from reduced capacity with repeated charge & dischargecycles.

The electrochemical battery stack was background charged when sufficientcharge had been accumulated on the input capacitor⁸. In a laboratorytest of vibration energy harvesting simulating low level vibrations thatmight be found on a ship or aircraft, the PZT harvester produced from2.2 to 2.8 milliwatts of output power at input vibration levels of only0.1 to 0.13 G's and at relatively low strain levels (150 to 200microstrain). At these low vibration levels the vibrations were barelyperceptible to human touch. FIG. 10 plots strain input vs. power outputfor both resonant flexure and non-resonant harvester types. Our wirelessrelative humidity and temperature demonstration node, programmed for aone second wireless update rate, may be powered perpetually with about100 milliG's of input vibration energy. ⁸ Arms, S. W., Townsend, C. P.,Hamel, M. J., Churchill, D. L., “Vibration Energy Harvesting forWireless Health Monitoring Sensors”, Proceedings Structural HealthMonitoring 2005, pages 1437-1442, Stanford, Calif., September 2005

A charge controller circuit was demonstrated that periodically checksthe state of the battery and, if appropriate, disconnects the load fromthe battery, thereby protecting the battery from damage which can besustained if the battery voltage drops below a prescribed voltage (2.0volts). This was accomplished by using a micropower comparator and a low“on” resistance switch. The quiescent current of this switch was lessthen 350 nanoamperes on average.

Inertial Sensing Suite Integrated with GPS

In one embodiment, a micro-electromechanical system (MEMS) inertial andmagnetic sensing suite, called 3DM-GX1™, is combined with a GlobalPositioning System (GPS) unit & antenna as shown in FIGS. 3, 4, and 8.3DM-GX1 combines three angular rate sensors with three orthogonal DCaccelerometers, three orthogonal magnetometers, a multiplexer, a 16 bitAID converter, and an embedded microcontroller, to output itsorientation in dynamic and static environments. Operating over the full360 degrees of angular motion on all three axes, 3DM-GX1 providesorientation in matrix, quaternion and Euler formats. The digital serialoutput can also provide temperature compensated, calibrated data fromall nine orthogonal sensors at update rates of 350 Hz. Output modes andsoftware filter parameters are user programmable. Programmed parametersand calibration data are stored in nonvolatile memory. Furtherdescription is provided in the '384 and '637 applications.

Inertial & Magnetic MEMS Sensing Suite

3DM-GX1 includes sensors 156 connected to signal conditioners andmultiplexer 158 for feeding data to microprocessor 160 which can runembedded software algorithms, as shown in FIG. 11 to compute orientationin Euler, matrix, and quaternion formats on board. Microprocessor 160 isable to store data on associated EEPROM 162. Also stored there aresensor calibration coefficients, orthogonality compensationcoefficients, temperature compensation coefficients, and digital filterparameters. The microprocessor can calculate Euler angles, quaternionand matrix as shown in box 164 and can provide output through RS 232, RS485, or CAN bus 166 to computer or host system 168 or to multidrop RS485 network 170. It can also provide 4 channel programmable analogoutputs.

3DM-GX1 uses its triaxial gyros to track dynamic orientation. It usesthe triaxial DC accelerometers along with the triaxial magnetometers totrack static orientation. The embedded microprocessor contains a uniqueprogrammable filter algorithm, which blends these static and dynamicresponses in real-time. The algorithm provides a fast response in theface of vibration and quick movements, while eliminating drift. Thestabilized output is provided in an easy to use digital format. Analogoutput voltages proportional to the Euler angles can also be provided.Full temperature compensation is provided for all nine orthogonalsensors to insure performance over a wide operating temperature range. Ablock diagram of the inertial sensing subsystem (ISS) is provided inFIG. 11.

3DM-GX1 Detailed Specifications

Parameter Specification Comments Attitude Range: Pitch, Roll, Yaw (°)360, all axes Matrix & Quatemion Modes +/−90, +/−180, +/−180 EulerAngles Mode Static Accuracy (°) +/−0.5 Dynamic Accuracy (° rms) +/−2.0Typical, application dependent Repeatability (°) +/−0.2 Resolution (°)0.1 General Performance A/D converter resolution (bits) 16 Turn on time(sec) 0.8 Analog output (Optional) 0-5 V 4 channels, user configurableUpdate Rate (Hz maximum) 100 Orientation outputs Physical Size (mm) 65 ×90 × 25 With enclosure 42 × 40 × 15 Without enclosure Weight (grams) 75With enclosure 30 Without enclosure Electrical Supply Voltage (V) 5.2 to12 DC Supply Current (mA) 65 Environmental Operating temperature (° C.)−40° C. to +70 With enclosure −40° C. to +85 Without enclosure Vibration(g rms) 4 20-700 Hz, white Operational Shock (g) 20 10 msec halfsineSurvival Shock (g) 500 Communications Serial Interface RS-232, RS-485RS-485 networking optional Serial Communications speed (kBaud) 19.2,38.4, 115.2 User selectable Angular Rate Range (°/sec) +/−300 Customranges available Bias Turn-on to turn-on repeatability (°/sec) TBD 25°C. fixed temperature In-Run stability, fixed temp. (°/sec) 0.1 After 15minute warm up In-Run stability, over temp. (°/sec) 0.7 Over −40° C. to+70° C. range Short term stability (°/sec) 0.02 15 second Allan variancefloor Angle random walk, noise (°/√hour) 3.5 Allan variance method ScaleFactor Error (%) 0.5 Over −40° C. to +70° C. range Nonlinearity (% FS)0.2 Resolution (°/sec) 0.01 G-sensitivity (°/sec/g) 0.01 Std w/g-sensitivity compensation Alignment (°) 0.2 Std w/ alignmentcompensation Bandwidth (Hz) 30 −3 dB Nominal Acceleration Range (g) +/−5Custom ranges available Bias Turn-on to turn-on repeatability (mg) TBD25° C. fixed temperature In-Run stability, over temp. (mg) 10 Over −40°C. to +70° C. range Short term stability (mg) 0.2 15 second Allanvariance floor Noise (mg/√Hz rms) 0.4 Scale Factor Error (%) 0.5 Over−40° C. to +70° C. range Nonlinearity (% FS) 0.2 Resolution (mg) 0.2Alignment (°) 0.2 Std w/ alignment compensation Bandwidth (Hz) 50 −3 dBNominal Magnetic Field Range (Gauss) +/−1.2 Bias Turn-on to Turn-onrepeatability TBD (mGauss) In-Run stability, over temp. (mGauss) 15 Over−40° C. to +70° C. range Noise (mGauss/√Hz) TBD Scale Factor (%) 0.7%Nonlinearity (% FS) 0.4 Resolution (mGauss) 0.2 Alignment (°) 0.2 Std w/alignment compensation Bandwidth (Hz) 50 Nominal

GPS Enhancement

Commercially available GPS units and antennas are available from a widevariety of sources. GPS antenna with external mount for external mounton an aircraft are environmentally sealed and are available fromNavtech, Model 12700 Antenna.

GPS data is used to compensate for inertial errors that can occur duringsustained aircraft turns. GPS input is relatively low cost to implement(from a parts perspective), and provides benefits, including veryprecise velocity data, ground speed, altitude, latitude, longitude, andtiming information⁹. The velocity data is used to correct 3DM-GX1orientation errors due to centrifugal forces. ⁹ El-Sheimy, N: Report onKinematic and Integrated Positioning Systems, FIG XXII InternationalCongress, Washington, D.C. USA, Apr. 19-26 2002(htt)://www.fig.net/nub/fig_(—)2002/TS5-1/TS5_(—)1_elsheimv.pdf)

Base station 102 for the structural health monitoring system supportsdata recording and remote access from both wireless and hard-wirednetworked sensors, as shown in FIGS. 4 and 12. Base station 102 servesas the data aggregation engine for the system, acquiring data fromhardwired, high speed networked sensors, such as the inertial sensingsubsystem (ISS) as well as data from the wireless sensing network(Wireless sensing node), such as the energy harvesting, load sensingpitch link. FIG. 12 provides a representative enclosure 102′ for basestation 102, along with those components integrated within thatenclosure, including IEEE 802.15.4 transceiver and cellular connectivity108, flash EEPROM 106 for data logging, on board DSP 104 with structuralusage algorithms, cockpit notification display 110, can bus 90 andvehicle bus 114. Base station 102 can be located on the helicopter or itcan be located on a ground vehicle that communicates with the helicopteror it could be in a hand held device where it could be used to query thewireless network.

Microprocessor engine DSP 104 can be a low power PC 104 compatiblesingle board computer based on the Intel® IXP425 XScale® networkprocessor, as shown in FIG. 12. The IXP425 is an implementation of theARM compliant, Intel XScale microarchitecture combined withcommunication peripherals including, 2 high speed Ethernet MACs,hardware accelerated cryptography, 2 high speed serial ports, a localPCI interface and DMA controller. Table III provides a set ofspecifications for the gateway.

TABLE III Gateway Specifications Parameter Typical Units Operatingtemperature range −55 to +85 Degrees C. On board temperature −55 to 85  Degrees C. measurement range Humidity range 0 . . . 100 % RHMicroprocessor Engine PC104 IXP425 n/a Xscale SBC Data securityencryption DES, 3DES, AES, with n/a tamper detection inputs InternalNon-Volatile Storage Compact Flash interface n/a (8 Gb max) HardwiredBus Interface CAN Bus 2.0 n/a Alternate Hardwired Network Ethernet(TCP/IP)/ n/a USB/Serial Interface Ethernet port supports hardwareaccelerated cyptography Wireless Sensor RF Data IEEE802.15.4 n/aInterface RF Transmission Frequency 2.450-2.490 GHz RF channels 16 n/aRF Transmission range 70 Meters (line of sight) RF Output power 0 dBm RFModulation Type Direct Sequence Spread n/a Spectrum Power (Fulloperating mode) 3.5 Watt Power (Sleep Mode) <5 mW Dimensions PC/104 fromfactor 3.8″ × 3.6″

Timing & Communications Protocols

Two communication interfaces allow transmission of real time sensor dataor downloading of previously recorded data. The first communicationsinterface is hardwired controller area network (CAN) bus 90. This CANbus may support the inertial sensing suite (ISS) interface to basestation 102. Alternatively a low power wireless interface can be used tosupport the many applications where it is difficult to impossible toembed lead wires for data communication from the system under test. Alow power IEEE802.15.4 bidirectional direct sequence spread spectrumradio link 108 and an embedded protocol stack that can support ad hocmulti-hop communications may be used in each of the wireless sensingnodes for these applications.

CAN Bus Hard-Wired Network

The wired bus network uses automotive grade, commercial off the shelf(COTS) CAN transceivers to provide a multidrop distributed communicationbus that allows up two 32 individual inertial sensing nodes to belocated on the network. The CAN bus is a broadcast type of bus. Thismeans that all nodes can “hear” all transmissions. There is no way tosend a message to just a specific node; all nodes will invariably pickup all traffic.

The CAN hardware, however, provides local hardware filtering so thateach node may react only to messages intended for the particular node.The network uses a 2 wire communication topology with a maximum datarate of 1.0 Mbps. The CAN bus may be converted into the aircraftstandard 1553 network protocol in the future. Note that when the wiredbus is used for the hardware communications architecture, then power forthese sensing nodes would also be provided on the network, whicheliminates the need for energy harvesting on these nodes. The CANnetwork can also support future nodes, which may include both sensingand actuation capabilities. Actuators can be used in concert withpiezoelectric materials for active damping and/or for providing signalfor non-destructive material testing (such as acoustic crack detection),as described in commonly assigned copending U.S. patent application Ser.No. 11/368,731, “Miniature Stimulating and Sensing System,” (“the '731application”) to John Robb et al, filed Mar. 6, 2006, incorporatedherein by reference. Actuators may also be used for damping vibration.

IEEE802.15.4 Wireless Network

The IEEE802.15.4 network is a standard for low power data communicationnetworks. These radio systems use extremely low power relative to radionetworks such as Bluetooth (IEEE802.15.1) and WiFi (IEEE802.11), andsuch are very suitable for use in distributed sensor networkapplications. The 802.15.4 radios use low power (1 mW) direct sequencespread spectrum (DSSS) radios at 2.4 GHz for the physical communicationlayer. The radio standard also incorporates AES128 data encryptionstandard for its security layer, which allows for secure transfer ofover the air data. The over the air data transfer rate is 250 kbps whichis adequate for transfer of stored data in this application.

The 802.15.4 standard does not specify the network topology to be used.However, since the radios are very low power, mesh network topologies,as shown in FIG. 13, are often implemented using this technology. A meshnetwork allows for any node in the network to transmit to any other nodein the network within its radio transmission range. This allows for whatis known as multihop communications, that is, if node 180 a wants tosend a message to another node 180 e that is out of radio communicationsrange, it can use an intermediate nodes 180 b, 180 c, and 180 d toforward the message to the desired node 180 e. This network topology hasthe advantage of redundancy and scalability. If an individual nodefails, such as 180 b, a remote node such as 180 a can still communicateto any other node in its range, which in turn, can forward the messageto the desired location by routing, for example, through nodes 180 f and180 g. In addition the range of the network is not necessarily limitedby the range in between single nodes, it can simply be extended byadding more nodes to the system. This multihop capability can be used inembedded instrumentation applications, where radio range can be degradeddue to fading losses and multipath interference when radios are embeddedin equipment.

Alternatively, when higher packet rates are desired, a star networktopology is more desirable, as shown in FIG. 13. The wireless nodes ofthe present patent application will support both a star and mesh networktopology. FIG. 13 shows a combination of star and mesh topology. Singlehop node 182 a and 182 b communicate only to multihop node 180 a forexample. The specific use will determine which topology is mostappropriate.

A single chip IEEE802.15.4 CMOS radio can be used for datacommunications and a third party multihop radio stack can be used toimplement the data communication protocol (MicroChip, Chandler, Ariz.).

Data logging transceivers with sensors for measuring differentparameters, such as acceleration, strain, and voltage, are availablefrom Microstrain, Inc., Williston, Vt. These devices employ addressablesensing nodes with data logging capabilities, embedded processing andcontrol, a bi-directional, direct sequence spread spectrum (DSSS) radiofrequency (RF) transceiver communication link, and rechargeable Li-Ionbatteries. For example, MicroStrain's commercially available SG-LINKsingle and multi-channel wireless strain sensing nodes(http://www.microstrain.com/sg-link_specs.aspx) include featuresdesigned to facilitate use with quarter, half, and full bridge foilstrain gauges as well as full bridge Wheatstone bridge transducers(pressure sensors, accelerometers, load cells, torque cells), as well asstrain rosettes. A full bridge strain gauge implementation can be usedon the pitch link to provide an output proportional to pitch link axialloads while compensating for thermal expansion and contraction.

The SG-Link features include on-board precision bridge completionresistors, wireless shunt calibration capability, wirelessly softwareprogrammable offset and gain adjust, wirelessly programmable samplingrates (currently from 1 to 2000 Hz), and multiplexed & pulsed straingauge bridge excitation (to enhance battery life). The thermal stabilityof these systems (from 0 to 50 degrees C.) is 0.007%/deg C. (offset) and0.004%/deg C. (gain). Strain measurement range is typically +/−2500microstrain full scale, with strain measurement resolution of +/−2.5microstrain (2 pole low pass filter 3 dB down at 500 Hz)¹⁰. Thesecapabilities also exist on wireless sensing nodes of the present patentapplication. ¹⁰ Arms et al., Wireless Strain Sensing Networks, 2ndEuropean Workshop on Structural Health Monitoring, Munich, Germany, 7-9Jul. 2004

A central host orchestrates sample triggering and high speed logging toeach node or to all nodes of a network of these wireless transceivers.Data may be processed locally (such as frequency analysis) then uploadedwhen polled from the central host. By providing each sensor node with a16 bit address, as many as 65,000 multi-channel nodes may be hosted by asingle computer. Since each node only transmits data when specificallyrequested, the power usage can be carefully managed by the central host.One embodiment of the base station included cellular telephonecapability for transmitting data and for remote programming and datamanagement of a strain sensing network. Embodiments of the presentpatent application add such functions as GPS, energy harvesting,precision time keeping, high speed data logging, and features for lowpower operation.

System Time Synchronization

With multiple physical interfaces for communications employed in thesame network, time synchronization is maintained between all nodes onthe network. Maintaining time synchronization between the gateway andthe CAN inertial nodes is relatively simple, as the busses are wiredtogether and synchronization can be easily maintained using the wiredbus. However, the wireless network nodes are more difficult to keepsynchronized because they are very low power devices that cannot affordthe energy to constantly listen for a synchronization packet. Therefore,it is desirable to only periodically send a timing packet to thewireless node for synchronization. For this to be effective, a highlystable local time reference is provided at each wireless node. Thepresent applicants found that a stable real time clock with temperaturecompensation for the clock achieves a timing stability of approximately2 parts per million (ppm). A 2 ppm error would result in a timing erroraccumulation of 2 microseconds per second. The present applicants foundfor a star network that the desired network timing synchronization of 1millisecond can be achieved by broadcasting a synchronization byte witha time synchronization update just once every 500 seconds.

The structural monitoring system (SMS) of the present patent applicationbreaks down significant barriers to structural monitoring. The scalablewireless network eliminates costly wire runs to strain gauges & othersensors. Energy harvesting eliminates the need for battery maintenance.Support for the high speed networked CAN bus standard supports power &data acquisition for inertial sensors, with less wire (since the CAN bussupports a multi-drop network). GPS support provides important vehiclevelocity & position data, and enhances the accuracy of the inertialsensing system (ISS).

The system architecture of the present patent application allows forflight tests to be performed with a range of wireless and networkedsensing nodes. The wireless nodes may be deployed to monitor the loadson the rotating components of helicopters, for example, but they mayalso be used for fatigue monitoring on fixed wing aircraft. Unmannedvehicles could also benefit from the capability to autonomously trackand assess structural damage “on the fly”.

Time stamped load data from key structural elements (collected by thewireless sensing nodes), combined with pitch, roll, and yaw information(collected by the ISS/GPS) is gathered by the on-vehicle Gateway. Thisallows time slices to be made through all the data, based on peaks inthe inertial data. This time sliced information can be used for flightregime recognition.

This regime recognition may be performed on the Gateway itself so thatthe vehicle becomes “self-aware” and can assess and record severity ofusage, and usage history. This information can be used for enhancedcondition based maintenance, by providing important information on thefatigue rates of the vehicle's structure and rotating components.Combined with component tagging & tracking, operators and maintenanceand repair organizations can use the SMS to automatically update thestatus of the life-limited parts according to FAR 121.380 (a)¹¹. Theinformation can also be used in health usage monitoring systems (HUMS).¹¹ El-Bakry, M., Component Tagging & Tracking—An Essential EnablingTechnology for Effective ‘Safe Life’ Structural Monitoring, Proceedingsof 5^(th) Intl. Workshop on Structural Health Monitoring, Stanford,Calif., Sep. 12-14, 2005

A very high speed, microelectronics module, called NANO-DAQ whichsupports both fast data acquisition and complex sensor excitation forembedded test & evaluation systems is described in the '731 application.NANO-DAQ's high speed data acquisition package has achieved a very smallform factor and is expected to find uses in a wide range ofapplications, including: impact testing, modal analysis, acousticwaveform analysis, and materials testing. It can be used in sensor nodesof the present patent application.

NANO-DAQ is capable of sampling millivolt level differential sensorsignals (such as from a Wheatstone bridge type accelerometer) at veryhigh speeds (1 Mega sample per second, maximum) for a short time period(1 millisecond maximum). The sample rate and sampling duration aresoftware programmable, by the end user, over the USB port. Non-volatile,embedded flash memory stores up to 1000 data points with 16 bitresolution. In addition to data acquisition, the embedded microprocessorand on-board digital to analog converter (DAC) were combined to createan arbitrary waveform generator (AWG) which provides high voltage (+20V) sensor excitation waveforms. Thus, in addition to sensing, the nodesof the present patent application can be used to provide acoustic andelectrical signals for materials testing.

Piezoelectric Energy Harvesting Strain Measurement System

A schematic diagram of the system is shown in FIG. 14. One or morepiezoelectric material is connected to Conn2. When piezoelectricmaterial is subjected to dynamic (changing) strain, an AC voltage isgenerated at connectors JH1 and JH2. This AC voltage is rectified andfiltered by full wave diode rectification bridges. Charge provided bythis voltage is stored in capacitors ch7 and ch8. The voltage (Vbat) onthe capacitors is then regulated and supplied as system power to therest of the circuit. Alternatively, if more power is available thenrequired by the circuit, the extra charge can be stored in battery BT2by switching on the battery charging regulator, UH1. Once the voltage onVbat is reduced to a value that is lower then that required to chargethe battery the regulator is turned off by the microprocessor. Themicrocontroller U3 controls the basic operation of the applicationcircuit. Periodically, the microcontroller wakes up and samples datafrom the strain gauges and transmits or logs the sampled data to flashmemory, U6.

Health Monitoring of the Strain Gauge Using Built in Test (BIT)

The magnitude of the voltage produced by the piezoelectric patchincreases as the amplitude of the dynamic strain level increases(referred to as the PZT peak to peak amplitude, or Apzt. This signal canbe used as a reference to compare to the dynamic component (referred toas the strain gauge peak to peak amplitude, or Asg) of the strainsmeasured by foil strain gauges. One way to track the health of thestrain gauges is to form a ratio of these two amplitudes:

Ratio=Asg/Apzt

This ratio may be recorded by the on board processor and recordedlocally (or logged on an on-board machine) during typical operatingconditions, such as straight and level flight. The ratio will tend toremain the same during operation if the health of the PZT and straingauge elements does not change. In the case of the strain gauge outputdegrading because of delamination (or debonding of the strain gauge fromthe component substrate), the ratio would tend to decrease. In the caseof the PZT elements debonding from the component substrate, or shouldsome of the PZT fibers start to fail, this ratio would increase.

One could also use the difference (rather than the ratio mentionedabove) between the strain amplitudes as measured by the piezoelectricand by the foil gauges as a built in self test (BIST). Ideally, adynamic calibration procedure would be used to allow the PZT amplitudeto be expressed in units of microstrain. This would require acalibration procedure, which could be done at the factory prior todelivery to the customer, in order to document the sensitivity of thePZT to applied dynamic strains.

The instrumented component could be calibrated for static and dynamicload measurement. Known loads would be applied through a dynamicactuator (such as a hydraulic ram) through a “gold standard” such as apre-calibrated load cell. The loads as reported by the load cell wouldbe monitored simultaneously with outputs of amplitude from the PZTelements as well as the strain gauges. This information would be storedin order to facilitate conversion of digital output from the electronicsthat condition the PZT and strain gauges into separately measured loads(dynamic for the PZT, and both static and dynamic for the straingauges). A static load calibration can be used to derive load bothstatic and dynamic information from the output of the strain gauges,while the PZT cannot be calibrated statically.

One method of calibrating the PZT output would be to use the dynamicstrain gauge output as the “gold standard” for strain measurement tocalibrate the PZT. This has the advantage of eliminating the need for adynamic load calibration. It may be advantageous to perform thiscalibration method early in the installation process, when the straingauges and the PZT have not been subjected to potential degradation fromexposure to the environment and from cyclic strain.

Piezoelectric materials can be used as strain gauges when the staticloads are not important or do not need to be measured. In the case ofthe pitch link, the static and the dynamic loads are significant, andboth need to be measured in order to obtain a reliable estimate offatigue of that structure. However, one could use a piezoelectric straingauge (as opposed to a foil strain gauge) to measure dynamic structuralstrains. The advantage of the piezoelectric strain gauges in systemswhich use energy harvesting systems is that no power is required for thesensing element. A separate piezoelectric element can be used to measurestrain, as opposed to using the PZT energy harvesting element to provideboth power and sensing functions. The reason for this is that the PZTused to provide power is generally a larger element, because it isdesigned to capture as much strain energy as is practical in theapplication, and therefore, is not well suited to discrete strainmeasurement locations. Furthermore, the PZT used for energy harvestingis loaded by other elements in the circuit, which may introduce someerror in the strain measurement. Therefore, in the case where dynamicstrain measurement only is required, an accurate and low power method ofaccomplishing this would be to substitute a piezoelectric strain gaugefor the foil strain gauge at the input to the amplifier (pins 2 & 3 ofconnector JP2)

Obtaining units of microstrain from a bonded foil strain gauge isaccomplished through the well known shunt calibration procedure, whichentails placing a known resistance across one of the strain sensingelements and measuring the static response of the microelectronics tothis shift in resistance. Provided that the sensitivity (gauge factor)of the strain gauges is known, and provided that the resistances of thebridge and shunt calibration resistance are known, one can calculate theamount of strain that is simulated by placing the shunt resistanceacross the strain gauge. This procedure allows the end user to convertfrom bits output from a strain sensor into physical units ofmicrostrain. The sensitivity of the strain gauge electronics can beperiodically tested using the built in shunt calibration facilities thatare built into the circuit. If, with periodic shunt calibration tests,the sensitivity has changed dramatically, that result would indicatethat some element in the amplification and signal conditioning chain hasbeen compromised.

Furthermore, the built-in offset capabilities of the strain gaugecircuit can be used to test for strain gauge stability. Therefore, thechange in apparent offset on the strain gauge can be monitored overtime. A shift in offset over time is likely an indication of eitherstrain gauge debonding, or moisture ingress into the strain gauge.

In quarter bridge systems, offset shifts of the strain gauge towardsincreasing resistance would indicate fatigue of the strain gauge'selements. Offset shifts in the direction of reduced resistance reflectpotential moisture ingress.

Moisture ingress into the strain gauge, PZT, and electronics elementscan also be detected using a thin, integrated capacitive moisturesensor, such as we have described in our previous patent application,incorporated herein by reference, and entitled “Strain gauge withmoisture barrier and self testing circuit”, by Arms et al., U.S. patentapplication Ser. No. 11/091244 filed 28 Mar. 2005.

One concern that could be raised is that the ratio (Asg/Apzt ) couldappear to indicate a healthy sensing system in the case where the straingauge is debonding or degrading while the PZT is simultaneouslydebonding or degrading. If these two separate elements should degrade inamplitude at the same rates, then the ratio (Asg/Apzt) would remain thesame, and the measurement system would appear to be healthy even thoughit would be degrading. This problem can be overcome in two ways. One waywould be to place the vehicle or aircraft in a known operating regime,such as straight and level flight at a relatively fixed airspeed andgroundspeed. Under these conditions, the strain gauge's peak to peakamplitudes and the PZT's peak to peak amplitude could be recorded. Ifthese amplitudes vary by some percentage below what was recorded afterinitial installation, then the system would indicate that degradationmay have occurred.

Alternatively, one could monitor the average energy output of the PZTelements, such as the voltage on the storage elements of the energyharvesting circuit (Vbat), which reflects the amount of energy that hasbeen collected by the system. Immediately after start-up, the systemcould take an initial measurement of Vbat. Then, after a programmableperiod of time, the system would make a second measurement of Vbat. Theamount of energy stored between these two time periods may be calculatedby the measured difference in voltages. The amount of energy consumedduring this measurement period would be known, based on previous testingof the system for the operating conditions (sensor load, electronicsload, harvester efficiency, etc.). If the average amount of energystored was reduced, as determined by these built-in-tests, performedover a long period of time, then this reduction in energy produced wouldreflect PZT fiber breakage (degradation) or it would reflect PZTdebonding from the substrate.

Another strategy for built-in-test would be to “ping” the PZT elementsmounted on the component with enough electrical energy to createmechanical response in the component and a measurable strain responsefrom the strain gauge. This response of the strain gauge could becharacterized and any significant reduction in its magnitude wouldreflect strain gauge debonding. The advantage of this method is that itcould be performed while the vehicle is not in operation. Thedisadvantage of this method is that energy must be supplied to to thePZT in order to create a mechanical response (strain) in the component.Methods for exciting or “pinging” a piezo element have been previouslydescribed in the U.S. patent application Ser. No. 11/368,731, “MiniatureStimulating and Sensing System,” (“the '731 application”) to John Robbet al, filed Mar. 6, 2006, incorporated herein by reference.

Measuring Loads in Rotating Components

Strain gauges can be used to convert a structural element into a load ormoment sensing element. In the case of the pitch link, four straingauges may be arranged around the pitch link's cylindrical shaft toamplify tension & compression while cancelling out thermal effects andbending loads. These techniques for instrumenting a column element as alongitudinal force sensing element are well known (reference:Measurements Group, Inc., “Strain Gage Based Transducers, Their Designand Construction”, pages 25-28, 1988).

The instrumented pitch link may be calibrated as a load sensing element.The easiest way to calibrate the instrumented pitch link would be toapply known loads using a reference load cell or weights. The knownloads are related to the pitch link's output in bits, and these data maybe stored locally (in the embedded electronics non-volatile memory)and/or remotely (on a nearby base station or on a server connected tothe internet). In either case, these data would allow the end user toconvert from bits output to actual working loads.

Application of Load Measurement for Condition Based Maintenance

The working pitch link loads during helicopter flight are very useful toacquire. These data can provide insight not only into fatigue of thepitch link, but to the overall severity of usage of the entirehelicopter. Previous work, using strain gauges bonded to the pitch link,and slip rings to power and acquire data from the rotating components,have shown that pitch link loads increase significantly depending of thehelicopter's flight regime. Since the pitch link is pinned to otherstructures (the pitch horn and the swash plate), the pitch link loadsalso provide insight into the loads borne by other structures on themachine. These connecting structures are also subject to cyclic fatigue,and their rate of fatigue depends on the severity of usage.

Packaging for the Energy Harvesting Wireless Pitch Link Sart Component

Our approach is to electromagnetically shield and environmentallyprotect our electronics assembly after direct epoxy bonding to pitchlink 200. Polyurethane materials have been used to surround and protectby providing a tacky, conformal seal for our microelectronics, whichprevents moisture ingress. Polyurethane material 202 is protected by athick overcoat of elastomeric shrink tubing 204. This method is ideallysuited to the pitch link application, as it secures the conformalpolyurethane sealant material and provides excellent mechanicalprotection of our electronics module 206, RF antenna 52, strain sensingelements 32, and energy harvesting elements 50 after mounting to aslender cylindrical element, pitch link 200, as shown in FIGS. 15 a, 15b.

The injectable polyurethane sealant (part number HT 3326-5, AviationDevices and Electronic Components, AV-DEC, Fort Worth, Tex.) wasoriginally designed for environmental sealing of electrical connectors.We determined that this type of conformal coating would be useful forenvironmental protection of the micro-electronics module, battery, andPZT harvester materials (after bonding of these elements to the pitchlink).

Flexible sealants 202 can also be silicone rubber or the HT 3326-5polyurethanes, and can provide good moisture protection, however, theyremain soft and tacky after curing, and therefore, they requiremechanical protection. Mechanical protection can be realized bysubsequent application of heat shrinkable tubing (or cold shrinktubing). Heat shrink tubing 204 may be obtained with an integralconductive electromagnetic interference (EMI) screening (or “shielding”)and/or hot melt adhesive (The Zippertubing Co., Los Angeles, Calif.).Zippertubing possesses an advantage over traditional heat shrink tubingbecause the pitch link rod ends 208 a, 208 b would not need to beremoved to provide environmental protection for pitch link electronics.

An alternative for EMI protection (to using heat shrink tubing withintegral EMI conductive screening) would be to use a pre-formed gasket210 of tacky polyurethane (also from AV-DEC) which includes an integralEMI shield 211. Printed circuit board (PCB) 206 may be designed toaccept this electrically conductive gasket 210 at an area where a groundreference may be made, and the pre-formed gasket 210 would be placedover this area, but with a layer of electrically insulative material 212(such as thin polyimide sheet) to prevent electrical shorting in thoseareas of the PCB that need to be insulated from shield 210. In this way,the PCB and other sensitive electrical components may be shielded fromEMI without shielding the radio antenna (which would be part of the PCB)and without causing electrical shorting to occur.

An alternative to heat shrink tubing has also been procured, this istermed “cold shrink” tubing (3M Aerospace & Aircraft MaintenanceDivision, St. Paul, Minn.). Cold shrink tubing possesses the advantageof maintaining a constant compressive load over time (i.e., the materialexhibits less creep and stress relation compared to heat shrink tubing).Cold shrink tubing, combined with the injectable polyurethane sealant,can provide excellent long term protection of the final pitch link loadsensing assembly, including the strain/load sensors, microelectronicsmodule with radio link, and energy harvester.

Additional protection from incidental impacts can be provided to theelectronics by placing two vertical strips 216 of a tough polymer, suchas solid polyamide or fiberglass, on either side of the slenderrectangular PCB board, as shown in the attached figure. The cold shrinkcovers the strips, the PC board, and the component.

While the disclosed methods and systems have been shown and described inconnection with illustrated embodiments, various changes may be madetherein without departing from the spirit and scope of the invention asdefined in the appended claims.

1. A method of maintaining a structure comprising, a. providing astructure having a component subject to component failure; b. mounting asensor, a memory and an energy harvesting device on the structure; c.using said sensor and logging data derived from said sensor in saidmemory, wherein said memory is powered solely with energy derived fromsaid energy harvesting device; and d. replacing said component ifinformation in said memory shows that said component was subject todamaging usage.
 2. A structure as recited in claim 1, wherein saidstructure comprises a vehicle.
 3. A structure as recited in claim 2,wherein said vehicle comprises an aircraft.
 4. A structure as recited inclaim 3, wherein said aircraft comprises a helicopter.
 5. A structure asrecited in claim 1, wherein said component comprises a rotating part. 6.A structure as recited in claim 5, wherein said component comprises ahelicopter pitch link.
 7. A structure as recited in claim 1, whereinsaid sensor includes at least one from the group including a straingauge and a piezoelectric transducer.
 8. A structure as recited in claim7, further comprising providing a programmable triaxial strain gaugesignal conditioner with integral self calibration.
 9. A method ofoperating a system as recited in claim 1, wherein said sensor includesat least one from the group consisting of an inertial sensing suite anda GPS.
 10. A structure as recited in claim 9, further comprisingproviding a processor.
 11. A structure as recited in claim 10, whereinsaid processor is connected for controlling operation of said wirelesscommunication device.
 12. A structure as recited in claim 10, furthercomprising providing a multiplexer connected to provide data derivedfrom a plurality of sensors to said processor.
 13. A method of operatinga system as recited in claim 9, further comprising using said processorto perform calculations and further comprising transmitting results ofsaid calculations.
 14. A structure as recited in claim 1, wherein saidmemory includes a volatile memory portion and a non-volatile memoryportion.
 15. A structure as recited in claim 1, further comprisingproviding data directly from said sensor to said volatile memory portionand then transferring said data to said non-volatile memory.
 16. Astructure as recited in claim 1, further comprising providing a lowpower time keeper and providing a periodic signal to said processor fromsaid low power time keeper for waking said processor from sleep mode.17. A method of operating a system as recited in claim 16, wherein powerto said processor is turned off during time between said periodicsignals.
 18. A method of operating a system as recited in claim 16,further comprising providing burst mode sampling data from said sensor.19. A method of operating a system as recited in claim 18, furthercomprising buffering data acquired in said burst mode sampling beforetransmitting said data on said wireless communication device.
 20. Amethod of operating a system as recited in claim 16, further comprisingan amplifier, wherein during said waking said processor said amplifieris kept out of saturation.
 21. A method of operating a system as recitedin claim 1, further comprising mounting a wireless communications deviceto the structure and transmitting data derived from said sensor withsaid wireless communications device, wherein all power for operatingsaid wireless communications device is derived from said energyharvesting device.
 22. A method of operating a system as recited inclaim 21, wherein said wireless communication device includes an802.15.4 transceiver.
 23. A method of operating a system as recited inclaim 1, further comprising providing a wired network for connectingsensors for logging to said memory.
 24. A method of operating a systemas recited in claim 1, further comprising wirelessly connecting sensorsfor logging data to said memory.
 25. A method of operating a system asrecited in claim 1, further comprising connecting an actuator to saidsensor module.
 26. A method of operating a system as recited in claim25, wherein said actuator is connected for providing a signal to saidstructure for material testing.
 27. A method of operating a system asrecited in claim 1, further comprising providing encryption to saidinformation before transmitting.
 28. A method of operating a system asrecited in claim 1, further comprising providing a warning to saidinformation before transmitting.
 29. A method of operating a system asrecited in claim 1, wherein said sensor module is one in a network ofsensor modules.
 30. A method of operating a system as recited in claim29, wherein said network of sensor modules is arranged in a meshnetwork.
 31. A method as recited in claim 1, further comprising arechargeable battery connected for recharging from said energyharvesting device.
 32. A method of operating a system as recited inclaim 1, wherein said damaging usage includes a load exceeding athreshold.
 33. A method of operating a system as recited in claim 1,wherein said damaging usage includes fatigue inducing cyclic loading.34. A method of operating a structure comprising: a. providing astructure having a component subject to component failure; b. mounting asensor module to said structure for measuring a parameter related tocomponent failure, wherein said sensor module includes a sensor, awireless communication device and an energy harvesting device; c.acquiring data with said sensor; d. providing information derived fromsaid data to said wireless communication device; e. powering saidwireless communications device solely with energy derived from saidenergy harvesting device; f. transmitting said information with saidwireless communication device; g. using said information to adjustoperation of said structure so as to avoid damaging usage.
 35. A methodof operating a system as recited in claim 34, further comprisingproviding a warning if said information shows that said component issubject to damaging usage.
 36. A method of operating a system as recitedin claim 34, wherein said damaging usage includes a load exceeding athreshold.
 37. A method of operating a system as recited in claim 34,wherein said damaging usage includes fatigue inducing cyclic loading.38. A method of operating a system as recited in claim 34, furthercomprising using said information to set a time for maintaining saidstructure.
 39. A method of operating a structure comprising, a.providing a structure having a component subject to component failure;b. mounting a sensor, a memory and an energy harvesting device on thestructure; c. using said sensor and logging data derived from saidsensor in said memory, wherein said memory is powered solely with energyderived from said energy harvesting device; and d. using information insaid memory to adjust operation of said structure so as to avoiddamaging usage.
 40. A system comprising, a network of sensor nodeswherein each said sensor node includes a sensor, a processor, a memory,a low power time keeper, a wireless communication device and an energyharvesting device, said processor connected to receive data derived fromsaid sensor, said memory connected for storing data derived from saidsensor, said low power time keeper connected to periodically provide asignal to wake said processor from a sleep mode, said wirelesscommunication device connected for communicating data derived from saidsensor, said energy harvesting device connected for harvesting energy topower said processor, said memory, and said wireless communicationsdevice.
 41. A method of operating a system comprising: a. providing asensor node including a sensor, a processor, a memory, a low power timekeeper, a wireless communication device and an energy harvesting device;b. providing a signal from said low power time keeper to the processorto power up said processor from a powered off or a low powered conditionat predetermined intervals of time; c. using energy derived from saidenergy harvesting device to provide power for operating at least onefrom the group consisting of said processor, said memory, and saidwireless communications device; d. providing data derived from saidsensor to said processor; e. storing data derived from said sensor insaid memory; and f. using said wireless communication device toexternally communicate data derived from said sensor.
 42. A method ofoperating a system as recited in claim 41, wherein power is turned offbetween said intervals of time.
 43. A method of operating a system asrecited in claim 41, wherein said sensor node is mounted on a structure,further comprising using data derived from said sensor to set a time formaintaining said structure.
 44. A structure, comprising a wirelessinstrumented structural component including a first sensor, a secondsensor, a processor, a transmitter, and an energy harvesting device,wherein said first sensor is for measuring a first property related tostructural load in said structural component, said second sensor is formeasuring a second property related to structural load in saidstructural component, wherein said first property differs from saidsecond property, wherein said transmitter is connected to provide loaddata for said structural component, wherein all power for operating saidtransmitter is derived from said energy harvesting device, wherein saidprocessor is connected to receive an output derived from said secondsensor for verifying operation of said first sensor.
 45. A structure asrecited in claim 44, wherein said first sensor includes a strain gauge.46. A structure as recited in claim 44, wherein said second sensorincludes a piezoelectric transducer.
 47. A structure as recited in claim46, wherein said energy harvesting device includes said piezoelectrictransducer.
 48. A method as recited in claim 44, further comprising arechargeable battery connected for recharging from said energyharvesting device.
 49. A structure as recited in claim 44, wherein saidstructural component comprises an aircraft part.
 50. A structure asrecited in claim 44, wherein said structural component comprises ahelicopter part.
 51. A method of using a structure, comprising a.providing a wireless instrumented structural component mounted to thestructure, said wireless instrumented structural component including afirst sensor, a second sensor, a transmitter, and an energy harvestingdevice, said first sensor for measuring a first property related tostructural load in said structural component, said second sensor formeasuring a second property related to structural load in saidstructural component, wherein said first property differs from saidsecond property; b. comparing data from said first sensor with data fromsaid second sensor to determine that said first sensor is operatingproperly; c. providing energy from said energy harvesting device whereinall power for operating said transmitter is derived from said energyharvesting device; and d. wirelessly transmitting data about structuralload in said structural component.
 52. A method as recited in claim 51,wherein said first sensor comprises a strain gauge and said secondsensor comprises a piezoelectric transducer.
 53. A method as recited inclaim 52, wherein said comparing includes a ratio of strain gaugeamplitude with piezoelectric transducer amplitude.
 54. A method asrecited in claim 51, wherein said strain gauge amplitude is a peak topeak amplitude and wherein said piezoelectric transducer amplitude is apeak to peak amplitude.
 55. A method as recited in claim 51, furthercomprising providing a moisture sensor, further comprising using outputof said moisture sensor to provide a check of said strain gauge data.56. A method as recited in claim 51, further comprising providing anenergy storage device connected for storing energy from said energyharvesting device.
 57. A method as recited in claim 51, furthercomprising measuring charge on said energy storage device.
 58. A methodas recited in claim 51, further comprising detecting a problem with saidenergy harvesting device from said charge on said energy storage device.59. A method as recited in claim 51, wherein said second sensor includessaid energy harvesting device.
 60. A method as recited in claim 51,further comprising providing a receiving device for receiving said data,wherein said receiving device receives data from a plurality of saidwireless instrumented structural components mounted to the structure.61. A method as recited in claim 60, further comprising determiningusage of the structure from said received data.
 62. A method as recitedin claim 60, wherein said receiving device is located on the structure.63. A method as recited in claim 51, further comprising providing arechargeable battery, and charging said rechargeable battery with saidenergy harvesting device.
 64. A sensing device, comprising an inertialsensor and a GPS, said inertial sensor integrated with said GPS whereinall power for operation of said inertial sensor is provided from energyharvesting.
 65. A sensing device as recited in claim 64 wherein saidinertial sensor includes a microprocessor, wherein said processor usesdata from said GPS to correct data from said inertial sensor.
 66. Asensing system comprising a base station, a first plurality of sensors,and a second plurality of sensors, wherein said first plurality ofsensors are connected to said base station on a wired network andwherein said second plurality of sensors are connected to said basestation on a wireless network.
 67. A sensing system as recited in claim66, wherein said wired network includes a CAN bus.
 68. A sensing systemas recited in claim 66, wherein said wireless network includes an802.15.4 network.
 69. A method of estimating time before failure of acomponent of a structure, comprising: a. providing the structure havingthe component; b. instrumenting said structure with a sensor and amemory, said sensor to measure a parameter related to the structure,said memory for logging data derived from said sensor; c. providing anenergy harvesting device on the structure, said energy harvesting deviceconnected to provide all power for powering logging data; d. providing amodel of the component subject to failure; e. entering informationderived from said data into said model; and f. using said model and saidinformation to estimate a parameter related to time before failure ofsaid component.
 70. A method as recited in claim 69, further comprisingproviding maintenance based on said estimated time.
 71. A method asrecited in claim 69, wherein said parameter related to the structureincludes a parameter related to input load experienced by the structureand wherein said using said mathematical model involves estimating aparameter related to time before structural component fatigue.
 72. Amethod as recited in claim 69, further comprising instrumenting saidstructure with a plurality of said sensors.
 73. A method as recited inclaim 69, further comprising instrumenting said structure with aplurality of said sensors.
 74. A method as recited in claim 69, whereinsaid sensor includes a strain sensor.
 75. A method as recited in claim74, wherein said model includes empirical data relating strain andcycles to failure.
 76. A method as recited in claim 74, wherein saidmodel includes an algorithm relating strain in one location to strain inanother location.
 77. A method as recited in claim 74, wherein saidmodel relates strain to load.
 78. A method as recited in claim 77,wherein said model includes an algorithm relating load in one locationto load in another location.
 79. A method as recited in claim 69,further comprising a wireless transmitter connected for transmitting atleast one from the group consisting of said data and said information.80. A method as recited in claim 79, wherein said data includes strain.81. A method as recited in claim 80, wherein said data includes timewhen a data point was logged.
 82. A method as recited in claim 79,wherein said information includes peaks and valleys of cycles of strain.83. A method of collecting information about a structure, comprising: a.providing an instrumented component including a sensor, a memory, and anenergy harvesting device, wherein said component is an integral part ofa structure when installed in the structure, wherein said instrumentedcomponent includes packaging to protect said sensor, said memory, andsaid energy harvesting device; b. installing said component in thestructure; and c. using energy derived from said energy harvestingdevice to provide power for logging data in said memory.
 84. A method asrecited in claim 83, wherein said packaging provides environmentalprotection.
 85. A method as recited in claim 83, wherein said packagingprovides shielding from electromagnetic field.
 86. A method as recitedin claim 85, further comprising an antenna extending outside saidshielding and inside said environmental protection.
 87. A method ofmaintaining a structure comprising, a. providing a structure having acomponent subject to component failure; b. mounting a sensor, a memoryand an energy harvesting device on the structure; c. using said sensorand logging data derived from said sensor in said memory, wherein saidmemory is powered solely with energy derived from said energy harvestingdevice; and d. logging data at a rate depending on amount of energyharvested by said energy harvesting device.
 88. A method as recited inclaim 87, wherein said rate depends on magnitude of strain experiencedby the structure.